1. Field of the Invention
This invention pertains to the field of reverse osmosis and ultra filtration. More particularly, the invention concerns a method of removing foulants from a used spiral-wound cartridge to bring it back to original production or near original production levels.
2. Description of the Prior Art
In the field of water filtration, most commercial processes involve the use of spiral-wound reverse osmosis cartridges. These cartridges are used either singly or in groups. The feed-water stream to the cartridge is either referred to as "feed-water" or as "brine" while the filtered water produced from the cartridge is called the "permeate" stream.
The cartridge is commonly constructed of one or more generally rectangular envelopes made of opposed sheets of a proprietary plastic film, sealed about three sides, that passes water molecules there across without passing other larger molecules, held apart by an interior layer of polysulfone and whose non-sealed edges are separately attached to the sides of an elongated slit formed in the wall of a centralized product tube. The envelope or envelopes are thereafter spiral-wound about the center product tube along with a layer or layers of an open web or screen called "brine channel webbing", to maintain the outer surfaces of a particular envelope free from direct contact with an overlying or underlying layer of membrane envelope, to a tight cylinder about the center product tube. Each end of the spiral wound cylindrical cartridge is capped with an element called an "anti-telescoping" device that keeps the spiral-wound layers from shifting lengthwise along the center product tube. Over top of the wound layers is placed either a shrink-wrap plastic in overlapping layers that is thereafter subject to heat or other shrink operation to form a tight, waterproof coating or is over-coated with a gel coat of curable, hardenable plastic either with or without a layer or layers of woven or non-woven fiberglass or other strengthening material.
In use, one end of the spiral-wound cartridge is capped to prevent water from exiting the layer of webbing, and the other end is connected to a source of feed water. The pressure of the feedwater is raised to approximately 200 pounds per square inch and by reverse osmotic pressure, the feed or brine water enters into the layer taken up by the brine channel webbing between the rolled envelopes of reverse osmosis material to force the water into the center of the envelopes where it moves by migration and pressure along the layer of polysulfone to enter through the slits into the center product tube and exit the cartridge as clear, fresh water. These cartridges range in size from as small as one inch to upwards of eight inches in diameter and in lengths of from less than two feet to over four feet in length. While in some cases, a single cartridge may be used, more often two or more cartridges are arranged in parallel to increase the permeate water output.
A significant problem has existed with the use of spiral-wound reverse osmosis cartridges namely that after an extended period of use, output rates of the permeate stream begin to subside and, if not treated, the cartridge will soon be rendered unusable. The reason for the decrease in output is because of the accretion of foulants brought in with the feed-water stream that enter the brine webbing, clog the webbing and coat the surfaces of the membrane envelopes.
There are generally three types of foulants; a carbonate based foulant such as calcium carbonate, a sulfate foulant such as calcium or aluminum sulfate, and sylicates such as aluminum and calcium silicates which precipitate from the feed-water. Past practices at removing these foulants are limited to flushing water-based cleaning solutions through the feed-water stream that soften or dissolve some of these foulants with respect to sylicates, their removal by this flushing practice has proven inadequate as they do not easily become solubilized. Another solution showing limited success is to add certain chemicals to the feed-water stream to keep the foulants in solution or suspension. This treatment has come under some criticism because the discharged feed-water stream, carrying these added chemicals, is passed into public sewage treatment plants and their presence sometimes interferes with sewage treatment or runs afoul of certain environmental regulations and statutes.
Some attempt has been made to remove the outer covering and anti-telescoping devices from the ends of the cartridge, unwrapping the cartridge and physically cleaning the individual membrane envelopes, however, no success has been achieved with rewrapping the envelopes and webbing into the tightly spiral-wound configuration as they were originally configured. Attempts to squeeze the wrapped envelopes and webbing into a tighter configuration has proved unsuccessful because the webbing tears the reverse osmosis membrane apart, thereafter allowing feed-water to pass directly into the permeate water stream. Modern treatment, therefore, has been to make one or two attempts to clean the cartridges using a chemical flush and, when that process proves to be ineffective, discard the cartridges and replace them with new cartridges. The cost of replacement cartridges, depending upon the size and use thereof, may be and often is a significant cost of operation that raises the volumetric price of the permeate stream.